Interpretive Summary: SWAT2005, a watershed-scale computer model, contains tile drain equations that have been successfully used to compute tile flow assuming tile drain systems are already designed with regard to tile spacing and size. Recently, new tile drain equations were incorporated into SWAT2005 (Modified SWAT2005) in order to provide an alternative tile flow simulation method which also allows for multiple scenario simulations, such as varying tile spacing, and drain tube size, in order to design cost-effective and environment-friendly tile drain water management systems for regions with shallow water tables such as the Midwest U.S. The objective of this study was to use measured streamflow data from the South Fork watershed (SFW) in Iowa to evaluate the capability of the Modified SWAT2005 to simulate streamflow for this large tile-drained watershed compared to SWAT2005. The study results showed that both models simulated streamflow fairly well with no significant differences in the simulated water balance components. This shows that the new tile drain equations within SWAT2005 have the potential as an alternative tile flow simulation method for areas with or without tile drainage system design information and also as tile drainage design tool within this widely used model.

Technical Abstract:
Although subsurface drainage is a water management system widely used to maximize crop production in regions with seasonal high water tables such as the Midwest U.S, it is also a major source of nutrients into water bodies. Recently, the SWAT2005 model was enhanced with physically-based Hooghoudt and Kirkham tile drain equations (herein referred to as Modified SWAT2005) in order to provide an alternative tile flow simulation method which can also be used to design cost-effective and environment-friendly tile drain water management systems. The goal of this study was to use measured streamflow data from the South Fork watershed (SFW) in Iowa to evaluate the capability of the Modified SWAT2005 to simulate streamflow for this large tile-drained watershed compared to SWAT2005. This was accomplished by comparing measured streamflow with that predicted by the two methods using the Nash-Sutcliffe efficiency (NSE) and percent bias (PBIAS, %) statistical methods in addition to hydrographs. During the relatively wet calibration periods, both models simulated the streamflow very well (monthly NSE =0.89 and PBIAS = ±9.5%) without significant differences in the simulation accuracy. During the relatively dry validation periods, both the SWAT2005 (monthly 0.53 = NSE = 0.68; ±0.7% = PBIAS = ±7.9%) and the Modified SWAT2005 (monthly 0.58= NSE = 0.72; ±0.0% = PBIAS = ±6.3%) models simulated streamflow satisfactorily. There were no significant differences in the water balance components simulated by the two models. This shows that the Hooghoudt steady-state and Kirkham tile drain equations within SWAT have the potential as an alternative tile flow simulation method for areas with or without tile drainage system design information and also as tile drainage design tool within this widely used model. Additional studies using measured tile flow to test flow simulation accuracy by both tile flow equations within SWAT are underway.